For the detection of ionizing radiation, in particular gamma radiation and X-radiation, scintillation detectors or direct-conversion radiation detectors based on semi-conductive materials are generally used. In scintillation detectors, the incident radiation is converted indirectly by electron excitation and conversion into photons of other wavelengths. These photons are then measured by photodetectors, whose output signal is a measurement of the incident radiation. Direct-conversion radiation detectors are different in that they are able to convert ionizing radiation directly into a readable signal. By way of a special semi-conductive material, the direct-conversion radiation detectors can also count the number of individual photons.
For this purpose, direct-conversion radiation detectors typically comprise detector elements which, in addition to the radiation detection material used for the detection of ionizing radiation, comprise at least two metal contacts made from a suitable contact material for at least one anode and one cathode. The radiation detection material and the contact material should therefore each have a specific charge carrier excitation energy level, and there should ideally be an ohmic contact on the boundary layer between both materials. This is because the radiation detection material is connected electro-conductively through the electrodes (i.e. the anode and the cathode) to the readout electronics and power supply of the detector via the metal contacts (e.g. platinum or gold contacts).
Previous direct-conversion radiation detectors have always been based on radiation detection materials made from semi-conductive compounds such as CdTe, CdZnTe, CdZnSe and CdZnTeSe. However, in these radiation detection materials, particularly when subjected to X-radiation and gamma radiation with high flux densities, as are typical and necessary in, for example, computed tomography devices, a space charge region is formed in the radiation detector via slow holes and/or fixed charges that are typically linked to deep and so-called intrinsic impurities. These deep impurities (with a depth of up to half the band gap energy) can trap the charge carriers produced by radiation and recombine with them. The space charge region produced in this way, and the reduction in movement ability for all charge carriers creates a reduction in the externally applied electric field, thereby reducing the pulse height, such that a clearly lower radiation intensity level is suggested. This means that the spectrum is shifted to lower energy values. This effect is called polarization, which limits the maximum detectable flux of a direct-conversion radiation detector.
In addition to the changes made to the internal electric field by this polarization effect, the contacts in conventional systems are not, however, usually ideal ohmic contacts, since they usually exhibit different carrier excitation energy levels at the boundary layer between the metal contact and the semi-conductive material. This is affected by various separation methods, such as hole-injection for platinum contacts and electron injection for indium contacts. This space charge region produced by the applied metal contacts also interferes with the separation and removal of the charge carriers produced by irradiation. The detector properties are also changed by the changing charge carrier transport properties. The space charge therefore intensifies the polarization effect.
To reduce polarization, the voltage applied externally to the detector can be increased. This does not, however, prevent the electric field change in the semiconductor. In order to minimize polarization, an attempt has also been made to produce an electrically conductive transition between metal and semi-conductive material, ideally an ohmic transition between metal and semi-conductive material, so that the charge carriers can pass through the boundary layer without a large amount of resistance.
For this, DE 10 2009 018 877 A1 describes, for example, an X-radiation detector, particularly for use in a computed tomography system, containing a semi-conductive material and a contact material, which each have their own specific charge carrier excitation energy. This X-radiation detector is characterized in that the excitation energy of conventional contact materials (e.g. Pt, Au, Ir or Pd) corresponds to the excitation energy of the semi-conductive material (e.g. CdTe, CdZnTe, CdTeSe and CdZnTeSe).
However, for certain applications, conventional contact materials are inadequate. This is particularly the case when the direct-conversion semi-conductive materials comprise a larger proportion of foreign ions and a reduced number of free charge carriers, for example to match the radiation converter to an application-relevant energy field.